1. Field of the Invention
The present invention relates to an optical system (and method) for diffusing light, and relates particularly to, an optical system having a diffractive element and a diffusing element, which diffuses light diffracted from the diffractive element. The invention is especially suitable for diffusing light for a projection television (TV) or display screen, or diffusing light for a window, skylight, light bulb or light tube, such that the diffused light has a uniform intensity distribution over a wide angle.
2. Background of the Invention
Optics for diffusing light is typically used for large screen projection TV""s, and includes a lenticular array consisting of vertically oriented cylindrical lenslets formed in a plastic sheet. The array distributes the light horizontally by an angular amount determined by the numerical aperture of the individual lenslets. Typical commercially available screens with a lenticular array have poor efficiency, 30% or less, and have undesirable color banding and white and dark lines at the edges of the pattern due to the diffraction effect of the lenslet array. Often a two-sided lenticular screen is used for projection TV""s having a black absorbing stripe between lenslets to increase the screen contrast and reduce ambient room-light reflections. One proposed design described in R. J. Bradley, J. F. Goldenberg and T. S. McKechnie, xe2x80x9cUltra-wide viewing angle rear projection television screen,xe2x80x9d IEEE Trans., Consum. or Electronics. CE-31, p.185-193 (1985), incorporates a complex lenticular surface, a bulk diffuser and black striping. The screen is reported to have a nearly uniform luminance pattern over a xc2x190xc2x0 range giving rise to an intensity distribution that falls off as a cosine of the scattering angle. The bulk diffuser is used to spread the light vertically. In this design, like in the typical projection TV, the diffusing optics do not use diffraction in multiple diffraction orders to enhance the diffusing of light.
Diffusing of light is also used for skylights. Typically, the skylights have a clear window pane installed at roof level of a room followed by a deep well that is painted white. The deep well acts as a diffusing reflector to prevent direct sunlight from reaching the room. Often when large skylights are used, special shaped diffusing reflectors are installed, such as in the Musxc3xa9e d""Orsay in Paris, France. While diffusing reflectors are effective, they are expensive and unaesthetic. Accordingly, it would be desirable to diffuse light from a skylight without the need for a diffusing reflector.
Diffusing of light may also be provided by frosting of glass used for windows, or in the area of artificial light, by the frosting of light bulbs. However, such light diffused through frosted glass is often not as uniform as desired.
The present invention relates to an optical system for diffusing light for use in illumination or display applications having a cascade of a diffractive element, such as a grating, and a thin diffusing element, referred to as a diffuser. Although cascades of a grating and a diffuser have been proposed in other areas, they have been limited to the analysis of coherence properties, such as discussed in K. M. Jauch and H. P. Baltes, xe2x80x9cCoherence of radiation scattered by gratings covered by a diffuser. Experimental evidence,xe2x80x9d Optica Acta 28, 1013-1015 (1981), detecting gratings hidden by diffusers, such as discussed in D. Newman and J. C. Dainty, xe2x80x9cDetection of gratings hidden by diffusers using intensity interferometry,xe2x80x9d J.Opt.Soc.Am. A 1, p. 403-411 (1984), or for use with wireless communication systems, such as discussed in E. Simova and M. Kavehrad, xe2x80x9cLight shaping diffusers for indoor wireless infrared communications via a holographic approach,xe2x80x9d in Diffractive and Holographic Optics Technology III, 1. Cindrich and S. H. Lee, ed., Proc. SPIE 2689, 284-291 (1996). The prior art cascades of a grating and a diffuser do not produce over a wide angle the uniform diffused light needed for illumination and display applications.
It is the principal object of the present invention to provide an improved optical system for diffusing light uniformly over a wide angle.
Another object of the present invention is to provide an improved optical system for diffusing light for projection TV""s with greater efficiency than the prior art lenticular screens, and can easily replace such lenticular screens.
A further object of the present invention is to provide an improved optical system for diffusing light which can be used as, or in combination with, windows or skylights, without requiring diffusing reflectors.
Yet a further object of the present invention is to provide an improved optical system for diffusing light produced by a display screen.
Another object of the present invention is to provide an improved optical system for diffusing light which utilizes a diffractive element for providing multiple diffraction orders.
A still further object of the present invention is to provide an improved optical system for diffusing light which can be placed on two sides of a single sheet of light transmissive material.
Briefly described, the optical system embodying the present invention includes a diffractive element for diffracting light received by the system in multiple diffraction orders, and a diffusing element which diffuses the diffracted light from the first optical element in accordance with the diffraction orders to provide diffused light.
The diffractive element provides diffracted light to the diffusing element having an intensity distribution in the form of a number of individual beams, termed diffraction orders, that are spaced by an angular separation determined by the diffraction period of the diffractive element and the illuminating wavelength or wavelengths. The intensity distribution of the diffracted light is correlated with the angular dependent intensity of scattered light provided by the diffusing element, which may be called the power spectrum of diffusion, such that the combination of the diffused light from each diffraction order provides a substantially uniform intensity over an observation zone or plane.
The correlation of the intensity distribution of the diffracted light to the diffusing element is such that the angular separation between the zeroeth diffraction order and the first diffraction order is approximately equal to one-half the angular width of the power spectrum of the diffusing element. The angular width represents the full-width-at-half-maximum of the power spectrum of the diffusing element. The strengths (intensity) of the diffraction orders are selected such that the combination (sum) of the diffused light from each diffraction order provides uniformity in the intensity of the diffused light from the system in the far field. Thus, the intensity distribution of the diffused light in the far field represents a superposition of the individual intensity distributions from the diffusing element when illuminated at the angle provided by each diffraction order of the diffracted light weighted by the strengths of the diffraction orders. The weights of the strength of the diffraction orders are such that the diffused light from each diffraction order contributes to provide a uniform combined intensity distribution of diffused light over a wide angle, such as between 45 and 100 degrees.
The diffractive element may represent a diffraction grating. The structure of the diffraction grating may be one-dimensional or two-dimensional. The diffusing element may have a surface which provides scattering at small angles (less than 10 degrees) of light incident on the diffusing element. The diffractive and diffusing elements may either be spaced a distance from each other, or integrated on a single body, such as on the two sides of a sheet of light transmissive material.
The combination of diffusing and diffractive elements can provide diffused light having a uniform intensity distribution over a wide angle in one direction with a much smaller angle in the other, which is desirable for a projection TV or the like. The combination of diffusing and diffractive elements can also provide diffused soft-light, such as for window or skylight, by selection of an appropriate grating structure for the diffractive element. The present invention is also applicable to other illuminating or display applications to provide high efficiency diffusing of light, such as display screens, microfilm viewers, greenhouses, illuminating signs, billboards, light bulbs and light tubes.